| As a typical kind of one-dimensionnal carbon materials, carbon nanotubes (CNTs) are widely used in nanoelectronic devices, composite materials, biomedical materials, and hydrogen storage materials, due to their unique electromagnetic, mechanical, thermal, optical and others properties. In these years, great achievements in teams of the preparation, growth mechanism and applications for CNTs have been gained, but there is not a viable way to prepare CNTs with controllable layer structure and high purity. Therefore, a new viable preparation process is extensively needed.In this paper, special multiwalled carbon nanotubes were synthesized by charring activated solid carbon mixed with a catalyst in the presence of a certain amount of co-catalyst in the carbonization furnace. Using SEM, TEM, HRTEM, XRD, Raman and other measurements, we found that the obtained carbon nanotubes have a diameter of 50-100nm, length of 0.2～4μm, thin wall, linear, preferable crystallization and the higher internal filling rate of catalyst. During the experiment, by changing the type and size of the carbon source, the type of catalyst and the mode mixed with the carbon source, acidification time, reaction temperature, holding time, the addition amount of H2O and thiophene (C4H4S) and other parameters, we ultimately obtained the best synthesis condition. We choose graphene oxide from 10～20μm natural graphite oxidated 24h by modified Hummers method as a carbon source, NiO/MgO as catalyst and carbonized the mixtrure of carbon source and catalyst with the mass ratio of 10:3 at 850℃ for 6-10h in the presence of steam and thiophene with the floe rates of 1.5ppm·min-1 and lppm·min-1, respectively, to get CNTs.Through the analysis of the experimental results, we obtain a new growth mechanism of CNTs, the process as follows:(1) Chemical oxidation introduces a large number of oxygen-containing functional groups and dangling bonds on the graphene layers, it benefits for adsorbing and co-anchoring the catalyst particles. (2) In the carbonization process, the catalyst is reduced by activated carbon and the reducing atmosphere, it become the molten metal state through adsording S, and agglomerate under the effect of Ostwald ripening. (3) With the catalyst particle continuing to etch the carbon layer and adsorbs carbon clusters, it makes the carbon concentration of the catalyst particle increase. (4) When the carbon concentration increasing of a certain amount, the carbon layer is deposited on the surface of the catalyst, due to the catalyst continuing S adsorption and etching carbon source, the catalyst is elongated and growed under the action of the expansion forces. (5) With S atom adsorption saturation and depletion of carbon, it will stop growing. Finally a kind of catalyst-coated carbon nanotubes are formed.When used as the anode material for lithium-ion batteries, the CNTs show high specific capacity and cycling stability. The initial discharge capacity is up to 957.3mAh-g-1 and the reversible capacity can maintain 600mAh-g-1 after 50 cycle times at 50mA·g-1. It is confirmed that some part of reversible capacities are from the metal sulfides of CNTs. How to increase the amount of metal sulfides in CNTs and further improve the electrochemical performance of materials are the next research direction in our laboratory. |
PDF Full Text Request